Electrocatalyst Potential and pH Effects

Introduction

An electrocatalyst's operating environment changes with applied potential and electrolyte pH. Electron-transfer driving force, proton availability, adsorbate coverage and even surface oxidation state can shift. A catalyst ranking reported at one potential and pH may not hold at another. To compare studies, the potential reference scale, bulk and local conditions, and product analysis must be explicit.

Core explanation

Changing electrode potential changes the chemical potential of electrons. For an elementary step that consumes an electron, a more reducing potential can make the step thermodynamically more favourable under a specified convention, though kinetic barriers and coverage need not change in a simple proportional way. Surface redox states and ion adsorption can also shift. A voltammogram therefore reflects coupled surface and solution behaviour rather than one isolated binding energy.

Proton-coupled electron transfer depends on proton activity as well as electron potential. In aqueous systems, SHE is a fixed reference definition, while RHE tracks the reversible hydrogen potential at the solution pH. At 298 K, the RHE potential is approximately 0.059 V per pH unit more negative than SHE under idealised conditions. Thus E versus RHE = E versus SHE + 0.059 × pH at 298 K for the same physical electrode potential. State the scale and pH before subtracting or comparing values. A JACS analysis of proton-coupled electron transfer explains why the RHE scale is useful for such processes.

Bulk pH may differ from local pH within the catalyst layer. Reduction reactions can consume protons or generate hydroxide near a high-current electrode faster than transport replenishes them. Buffer capacity, stirring, electrode geometry and current density influence the gradient. The local change alters reactant speciation, competing hydrogen evolution and adsorbate coverages. A nominal pH 7 electrolyte does not prove that every active site experiences pH 7. Measured reference conversion accounts for bulk pH unless local values are independently known.

Comparison also requires resistance correction and mass-transport control. An applied potential includes an ohmic drop in electrolyte or contacts, so the catalyst surface may see a different potential if uncompensated resistance differs. Current per geometric area mixes intrinsic rate with roughness and site count. Faradaic efficiency reveals where charge goes, while partial current to a product reports its rate-like electrical contribution. These quantities should accompany an activity claim.

Step-by-step reasoning

1. Specify the reaction and relevant proton/electron stoichiometry. 2. Report potential scale, temperature, bulk pH and reference calibration. 3. Check local pH or estimate its sensitivity to transport and current. 4. Measure product-specific currents and surface state at comparable conditions. 5. Interpret ranking only within the measured electrochemical window.

Visual explanation

Draw a potential axis labelled both SHE and RHE at pH 0 and pH 7, with the RHE zero shifting relative to SHE. Next draw an electrode covered by a thin diffusion layer: bulk pH on one side and a possibly different interfacial pH at the surface. Arrows show protons, buffer species and hydroxide moving between them.

Real-world analogy

Water pressure at a city reservoir differs from pressure at a tap if pipes cause losses and demand is high. Bulk electrolyte conditions resemble the reservoir; the reacting electrode interface resembles the tap. A voltmeter's reference scale is like the chosen pressure zero. The analogy helps separate reference and local conditions, though electrochemical potential combines electrical and chemical terms.

Real-world example

Two labs report hydrogen-evolution current at “−0.2 V,” but one uses SHE at pH 0 and the other RHE at pH 14. Those numbers do not specify the same driving force until reference and pH are accounted for. Even after conversion, differences in electrolyte resistance and bubble removal can change measured current. The labs calibrate references and compare product-specific kinetic currents, not the bare voltage labels.

Why?

Why can local pH change selectivity even if bulk pH is fixed? Product formation at a high-rate interface changes nearby proton and hydroxide concentrations. Since parallel reactions can depend differently on those species, their relative rates change. Transport and buffer capacity then become part of catalyst performance.

Common misconception

“Potential values are directly comparable without a reference electrode” is false. “RHE conversion removes every pH effect” is false because kinetic barriers, proton donors and local conditions can still differ. “Bulk pH equals active-site pH” can fail at high current. “A large total current proves high desired-product activity” ignores competing reactions.

Worked example

At 298 K and ideal conditions, an electrode held at −0.30 V versus SHE in a solution of pH 7 corresponds to approximately −0.30 + 0.059 × 7 = +0.113 V versus RHE. An electrode held at −0.30 V versus RHE at pH 7 is instead approximately −0.713 V versus SHE. The two written “−0.30 V” values describe very different physical potentials because their reference scales differ. Exact conversion uses the appropriate calibrated reference and temperature. Neither numerical conversion predicts reaction rate without kinetics and local conditions.

Quick check

1. Why must an electrocatalyst potential always include its reference scale? Answer: The numeric voltage is a difference relative to that reference; different scales have different zero points.

Exam focus

State how potential and proton activity enter a proton-coupled reaction. Convert SHE and RHE approximately at 298 K with pH stated. Explain local versus bulk pH and why product-specific current matters more than total current for selectivity.

Advanced insight

Some electron-transfer barriers respond to interfacial electric field and solvent orientation, not just a simple thermodynamic electron term. Specific adsorption of cations or anions can alter local field and intermediate stability. Therefore an identical RHE potential in two electrolytes need not yield identical kinetics; reference conversion is necessary but not sufficient for a fair comparison.

Summary

Potential and pH shape electrocatalytic energetics, surface coverages and product selectivity. Fair comparisons require stated reference scales, calibration, electrolyte composition, interfacial transport and product-resolved rates. Local conditions may differ from bulk measurements, so a voltage label alone cannot establish catalyst superiority.

Practice questions

1. At 298 K and pH 5, approximately what is 0 V versus SHE on the RHE scale? Answer: About +0.059 × 5 = +0.295 V versus RHE under the ideal conversion. 2. Can two experiments at the same bulk pH have different local pH? Answer: Yes. Current density, buffer and transport can create different interfacial gradients. 3. Why is total current insufficient to rank CO₂ reduction catalysts? Answer: It includes current to competing products such as hydrogen. 4. What can uncompensated resistance change? Answer: It causes an ohmic potential drop, so the surface potential differs from the nominal applied value.